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What I do (based on recommendation from other members of SAR) is to wash it after every use with mild soap, rinse it thoroughly, let it drip-dry overnight and then keep it in my freezer until the next time I use it, whether that’s in a day or a week. Pay particular attention to washing the nipple since there isn’t a lot of backwash but there is of course a lot of contact with your mouth and the elements.
I have seen a few problems with poorly maintained Camelbak hydrations systems, including at least one case where a Camelbak passed on food poisoning from one person to a group of people. What should I do to properly look after my Camelbak and to make sure it is safe to use?